There are known shot processing apparatuses for treating the surface of a coil spring workpiece by projecting projection material (see, for example, Patent Document 1).
In such apparatuses, in order to thoroughly project projection material over the entire circumference of a spring wire forming a coil spring, a coil spring is transported inside a blast cleaning chamber while being rotated about its longitudinal axis, and projection material is projected onto the coil spring using two centrifugal projectors. The two centrifugal projectors are arranged so that their respective blade wheels rotate in opposite directions, and so as to have differing peak projection angles in their projection distributions.